The present invention is related to a cordless headphone.
As a headphone stereophonic system, for instance, a cassette tape player is connected with a headphone by way of a cordless manner.
FIG. 1 is a perspective view for illustrating an outer view of one typical cordless type cassette tape player. In this drawing, reference numeral 1 shows a cassette tape player, and reference numeral 6 indicates a receiver exclusively used for this headphone.
Then, in the cassette tape player 1, an audio signal "R" of a right channel and an audio signal "L" of a left channel are reproduced from a cassette tape (not shown) in a stereophonic system during the reproducing operation. These right/left channel signals R/L are converted into FM signals SR/SL having preselected carrier frequencies, and then these FM signals SR/SL are transmitted to the receiver 6.
Then, in the receiver 6, when the FM signals SR and SL transmitted from the cassette tape player 1 are received, audio signals R and L are demodulated from these FM signals SR and SL, and then these audio signals R/L are supplied via a headphone cord 7C to right/left acoustic units 7R/7L of a headphone 7 so as to be converted into stereophonically reproduced sounds.
It should be noted that the distance over which the receiver 6 may be separated from the cassette tape player 1 during the stereophonic operation is within a range from 1 m to 3 m, for example, approximately 1.5 m.
FIG. 2 and FIG. 3 represents one example of signal processing systems for the above-described cordless type cassette tape player 1 and receiver 6. In the cassette tape player 1, the right/left-channel audio signals R/L are reproduced from a magnetic tape 2 of the tape cassette by reproducing heads 11L/11R during the reproducing operation. These audio signals R/L are supplied via reproducing equalizer amplifiers 12R/12L and preemphasis circuits 13R/13L to FM modulating circuits 14R/14L so as to be converted into FM signals SRT/SLT.
In this case, the carrier frequencies fRT/fLT of the FM signals SRT/SLT are selected to be, for example, EQU fLT=11.29 MHz EQU fRT=11.75 MHz.
Then, these FM signals SRT and SLT are supplied to a mixer circuit 15, and an oscillation signal S16 having a preselected stable frequency f16 (for instance, f16=249.75 MHz) is produced from an oscillating circuit 16. This oscillation signal S16 is supplied to the mixer circuit 15.
Thus, the FM signals SRT and SLT are added to each other in the mixer circuit 15, and then the added signal is frequency-converted by the oscillation signal S16. As a result, these FM signals SRT/SLT are frequency-converted into such FM signals SR/SL having the following carrier frequencies fR/fL: EQU fL=f16-fLT=238.46 MHz EQU fR=f16-fRL=238.0 MHz.
Then, these FM signals SR and SL are supplied via a bandpass filter 17 and an output amplifier 18 to a transmitter antenna 19 in order to be transmitted to the receiver 6.
On the other hand, in the receiver 6, the FM signals SR/SL transmitted from the cassette tape player 1 are received by the headphone cord 7C (namely, headphone cord 7C may be operated as an antenna), the received FM signals SR/SL are supplied via a bandpass filter 61 and a RF (radio frequency) amplifier 62 to a mixer circuit 63, and also a local oscillation signal S64 is supplied from a local oscillating circuit 64.
In this case, it should be understood that the frequency f64 of the local oscillation signal S64 is selected to be, for instance, EQU f64=248.7 MHz.
In this manner, these FM signals SR and SL are frequency-converted by the mixer circuit 63 by using the local oscillation signal S64 into intermediate frequency signals SRI and SLI having frequencies fRI and fLI given as, for instance, EQU fLI=fL-f64=10.24 MHz, EQU fRI=fR-f64=10.7 MHz.
Then, these intermediate frequency signals SRI and SLI are supplied via intermediate frequency circuits 65R and 65L containing filters and limiter amplifiers to FM demodulating circuits 66R and 66L, respectively, so as to demodulate audio signals R and L. These audio signals R/L are supplied to the "hot" sides of the acoustic units 7R and 7L via a signal line constituted by deemphasis circuits 67R/67L, output amplifiers 68R/68L, and RF choke coils 69R/69L. Also, at this time, the "cold" sides of the acoustic units 7R and 7L are connected through another RF choke coil 69G to the ground.
As a consequence, the audio signals R and L reproduced by the cassette tape player 1 can be heard by the headphone 7.
In this case, the dimension of the receiver 6 may be defined by, for instance, 50 mm (height).times.20 mm (width).times.10 mm (thickness). When the cassette tape music is played, since the cassette tape player 1 is connected to the receiver 6 in the cordless manner, while this cassette tape player 1 is stored in a bag, the receiver 6 may be put into a chest pocket of a jacket for example. Therefore, when the tape music is reproduced while the user goes to his office, or the user walks, there is no cord connected between the cassette tape player 1 and the receiver 6 to disturb the user operation.
Also, since the carrier frequencies fR/fL of the FM signals SR/SL transmitted from the cassette tape player 1 to the receiver 6 are selected to be, for example, fL=238.46 MHz and fR=238.0 MHz, namely higher than the frequency bands from 76 MHz to 222 MHz generally used in the FM broadcasting system and the television broadcasting system, these FM stereophonic signals SR/SL will not be adversely influenced by electromagnetic wave interference from broadcasting electromagnetic waves or reflections from surfaces in cities.
In the above-described cordless type headphone stereophonic system, the connection between the cassette tape player 1 and the exclusively used receiver 6 is made as a "cordless" connection, whereas the connection between the headphone 7 and this receiver 6 is not made as the "cordless" connection, but is established by the headphone cord 7C.
Therefore, it is conceivable to assemble the receiver 6 with the headphone 7 so as to eliminate the necessity of the headphone cord 7C. However, if so, then this headphone cord 7C can be no longer used as the reception antenna when the receiver 6 receives the FM signals SR and SL. Accordingly, another reception antenna must be employed.
As a result, for instance, as represented in FIG. 4, or FIG. 5, in order to obtain a high signal reception sensitivity, it is also conceivable to design this receiver 6 as a diversity reception system. In other words, in the receiver 6 shown in FIG. 4, the FM signals SR/SL are received by two sets of antennas 7A/7B and two sets of signal receiving circuits 6A/6B to derive two sets of audio signals R/L. These two audio signals R/L are supplied to a switch circuit 81. Also, reception levels of the receiving circuits 6A and 6B are detected by a detecting circuit 82 to output detection signals. A switch circuit 81 is controlled based on the detection signals.
In this manner, either the audio signal R, or the audio signal L, which is derived from the receiving circuit having the high reception level, is selected and derived from the switch circuit 7 among two sets of the audio signals R and L from the receiving circuits 6A and 6B. As a consequence, the better audio signals R and L can be continuously obtained irrespective of to the directional relationship between the cassette tape player 1 and the receiver 6.
Also, in the receiver 6 shown in FIG. 5, both the reception signal of the antenna 7A and the reception signal of the antenna 7B are switched at a frequency more than two times higher than the maximum frequencies of the audio signals R and L by a oscillation signal derived from an oscillating circuit 84, and then the switched reception signal is supplied to the receiving circuit 6A.
As a result, even when the level of one of the FM signals SR/SL received by the antenna 7A or the antenna 7B is low, if the level of the other FM signal is sufficiently high, then the better audio signals R/L can be produced from the FM signal received by the other antenna.
As previously explained, in accordance with the diversity reception type receiver 6, since the audio signals R/L produced from such an FM signal having the higher reception level among two sets of the FM signals SR/SL is used, the audio signals R/L can be obtained under better condition to reproduce better stereophonic sounds.
However, in the case of the diversity reception type receiver shown in FIG. 4, since a total number of circuit components is greatly increased, higher cost would be required. Also, since the total number of circuit components is increased, when this receiver 6 is assembled with the headphone 7 in an integral manner, the dimension of the headphone 7 would be increased, and further the weight of this headphone 7 would increase, which could impede the easy operation of the headphone.
On the other hand, in the diversity reception type receiver shown in FIG. 5, both the cost and the dimension of this receiver are not greatly increased, as compared with those of the diversity reception type receiver indicated in FIG. 4. However, since the audio signals R/L are partially derived from the FM signals SR/SL having the low reception levels, the S/N ratio and the clarity of the audio signals R/L supplied to the headphone 7 would be deteriorated.